What will it take for Celtics to advance past the Eastern Conference Finals?

After the Miami Heat eliminated the Celtics with a 125-113 win in Game 6 of the Eastern Conference Finals on Sunday, Boston coach Brad Stevens confirmed the obvious. The Heat are “the best team in the East and deserve to be representing the East in the way that they have played.”

But after appearing in three Eastern Conference Finals in the past four years, what are the Celtics to reach the NBA Finals?

“My regrets would be the outcomes, right? But my regret is not what we put in,” Stevens said. “My regret is not how we prepared or worked or how hard our guys worked or how committed they were to playing and staying together.”

The Heat relied on a resurgent superstar (Jimmy Butler), an emerging All-Star (Bam Adebayo), steady veterans (Goran Dragic, Andre Iguodala), a rising rookie (Tyler Herro) and an elite head coach (Erik Spoelstra).

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But the Celtics had the pieces too. They had a rising All-Star (Jayson Tatum), a versatile forward (Jaylen Brown), a veteran point guard (Kemba Walker), a shooting and defensive asset (Marcus Smart) and an elite head coach (Stevens).

And yet the Celtics spent most of this series imploding.

In Game 1, the Celtics squandered a 14-point, fourth-quarter lead in an overtime loss, and in Game 2, the Heat came back from 17 down. Boston controlled the fourth quarter of a Game 5 victory, then coughed up a six-point lead in the fourth quarter of Game 6.

“Just have to learn how to grow,” Walker said. “We’re going to have to figure out how to do those things down the stretch. We got time. At some point, we’re all going to look back at this and just learn. It’s all learning experiences.”

In the NBA, though, there is a thin line between staying patient with development and monitoring whether a championship window has closed. The Celtics lost in the Eastern Conference Finals in 2017 and 2018 mostly because the Cleveland Cavaliers had LeBron James, Kyrie Irving and Kevin Love. Boston lost in last year’s semifinals to Milwaukee partly because of Giannis Antetokounmpo and Irving’s poor fit with the Celtics’ young roster. This season, the Celtics had hoped to further develop their young roster (Tatum, Brown, Smart) while blending in a point guard who would better fit their style (Walker).

“My first year, there weren't too many expectations,” said Tatum, whom the Celtics selected No. 3 overall in the 2017 NBA Draft. “We were young. People didn't know what to expect. A couple guys were hurt. This year we had a lot of expectations.”

After making his first All-Star appearance, Tatum mixed in doses of brilliant and head-scratching moments. He averaged 26.5 points in the series, but he shot only 42.2 from the field and 26.4 percent from three-point range.

“It's not easy. It's tough,” Tatum said. “But if you want those expectations and you want to be that guy that's capable of doing those things, you got to go through some tough things, some up and downs, some stuff I can learn from. I feel I can learn a lot moving forward from this season, this series. I'll grow from it.”

Though the Celtics enjoyed continuity in their coaching staff and young core, they lacked stability elsewhere. Boston forward Gordon Hayward missed 12 games during the season restart after spraining his right ankle. Hayward only shot a combined 39.8 percent in four games of the series. Boston valued Walker’s positive attitude and hustle, but he never fully recovered from a left knee injury he suffered in January.

“It doesn’t matter, to be honest, at this point,” Walker said about his knee. “Anything I tell you will be an excuse. I was fine. I was able to be on that court.”

Boston overcame a locker room flare-up in a Game 2 loss, with Stevens saying, “We had one minor dust-up; that's pretty good for a calendar year with a group.” Throughout the series, though, Stevens lamented that the Heat played more physical than the Celtics.

“I thought our guys tried to play physical,” Stevens said. “I thought our guys tried to do that, but I think that ultimately it's more of a credit to them.”

The stakes might become even more difficult next season. The Bucks are expected to add more pieces around Antetokounmpo so he becomes enticed to sign a super-max contract. The Heat and Toronto Raptors will still have their core roster. The Brooklyn Nets will a field a healthy Irving and Kevin Durant.

“The league is going to be better. A lot of key guys were hurt this year. But I can't really answer that question,” Tatum said. “I don't know who is going to be on our team next year. But I believe in this group. I think we were capable. We had more than enough to get the job done. We just let a couple slip away. Got to give credit to them. They played great. But I really loved and enjoyed playing on this team.”

Nonetheless, that leaves the Celtics with plenty of work next season to ensure they can extend their championship window. 

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